In some parts of rural america, folks signal the presence of cops by flashing their headlights when driving.
Occurs to me that would be a cool function for SDR: press code for or say "cop". For N seconds, phone periodically sends "cop" message picked up by other phones, ignored by base station [1]. Phone also listens for these local broadcasts. Upon hearing a suprathreshold number of "cop" messages the phone alerts its owner. Better than a radar detector for emissionless, but visible, cops. Over the hill coverage. Issues of spoofing, trust, consensus familiar to readers here. [1] (different band entirely? UWB? B-tooth? FRS? )
